Field of the Invention
The sensitivity of electronic components and circuit boards to static charge requires the use of anti-static and static dissipative materials. Particularly in the areas where the static sensitive items are being tested, used, manufactured, packaged, and shipped, it is necessary to take steps to prevent the build-up of static charge. In order to protect these electronic components, desirably the materials in the environment should not hold static charge and should permit static electricity to be depleted gradually. It is thus particularly desirable to have antistatic or static dissipative environmental materials. Too little conductivity is undesirable and similarly, a high conductivity would likewise be undesirable.
Vinyl chloride polymers (both homopolymers and copolymers) have been very popular in surface covering compositions of all types. Floors, walls, counterspaces, etc. are frequently covered with decorative polyvinyl chloride sheet compositions. Polyvinyl chloride, however, presents particular problems if antistatic or static dissipative qualities are required. Polyvinyl chloride surface covering compositions without any anti-static additive will generally have surface resistivities of 10.sup.12 ohms/square (sq.) or higher. This surface resistivity is too high for the applications requiring either static dissipative (10.sup.6 to 10.sup.9 ohm/sq.) or anti-static (10.sup.9 to 10.sup.12 ohms/sq.) protection. Anti-stat additives are incorporated into materials in order to increase conductivity. Polyvinyl chloride, however, presents particular problems when the PVC must be combined with additives in order to achieve the anti-static and particularly the static dissipative conductivity levels.
Polyvinyl chloride surface covering compositions must contain a stabilizer so that they can be processed at elevated temperatures. Disadvantageously, the quarternary ammonium, salt cationic anti-stat additives tend to interfere with the heat stabilizing function of these metallic additives. Furthermore, discoloration of the polyvinyl chloride is noted when the cationic quarternary ammonium anti-stat and the metallic stabilizers are combined in the composition.
Alternatively, the vinyl formulations can be made using the non-ionic anti-stats. Unfortunately, this significantly limits the amount of conductivity which can be obtained. The non-ionic anti-stats do not improve conductivity as well as the cationic anti-stats.
Another difficulty which is frequently encountered with the vinyl formulations is the surface exudation of the anti-stat. Particularly when the formulations contain both plasticizer and anti-stat additive surface exudation often becomes a problem, especially with larger amounts of each (particularly needed to get static dissipative levels).
The instant invention provides polyvinyl chloride surface covering compositions having anti-static and static dissipative conductivity. The compositions provided are structured to optimize conductivity, but at the same time, problems of surface exudation are avoided. Even more desirably, the instant invention provides compositions which delivered excellent electrical performances, but at the same time, maintain good visual appearance. Compositions are even described herein which have both static dissipative properties and which have a decorative visual effect because of a printed layer in the composition.
The compositions described herein also combine several features which are used to provide lower surface resistivities. The instant sheet compositions can either be fixed (tacked, glued) to the surface or loosely placed on the surface as desired.